STRUCTURAL TIMBERS Timbers are usually sawed from the heart of the log. It pays the lumber manufacturer better to cut the clear, outside portions of the log into higher classes of material than it does to cut them into timbers which bring a lower market price. For this reason, timbers may contain many or all of the defects common to the species from which they are cut. Since, however, timbers are large pieces of wood which are used as a whole, some small defects do not greatly reduce the strength, and larger defects of certain kinds may not be serious unless located at the points where the greatest strength is required.
The most serious defects in structural timbers are rot, knots, shake, and cross-grain. Sometimes a beam or timber may be so placed that these defects will not seriously interfere with strength, 'whereas in a reverse position, they would be very detrimental. For example, knots near the center or ends have 'practically no effect upon the strength of a beam.
The rate of growth is often thought to have much effect upon the strength of large timbers; but they are so likely to have defects of greater importance that the rate of growth alone cannot be depended upon to indicate the strength. In the same way, while seasoning small sticks greatly increases their strength, it is not safe to assume that large timbers when seasoned are much stronger than when green. This is because checks which develop in seasoning are likely to offset the increase in strength due to the drying of the wood. For this reason, engineers do not ordinarily consider it advisable to figure upon a greater load for seasoned timbers than would be safe for timbers of the same size when green.
The Forest Service experiments in seasoning large timbers lead to these conclusions: (1) In general, timber 8 by 16 inches in cross-section must season through two entire summers before it reaches a thoroughly air-dry condition.
(2) The weight of thoroughly air-seasoned timbers will vary appreciably during the year, due to the alternate evaporation and absorption of moisture. This change in moisture content is accompanied by a corresponding shrinking and swelling which tends to increase the size and number of checks formed through the seasoning process. These hygroscopic changes, however, do not seem to affect the interior of the timbers.
(3) If seasoning is started in the hot summer months, the loss of moisture is at first very rapid, even though the timber is protected from the sun and wind. The rapid loss in weight is associated with a marked shrinkage in the outer portion of the timber, which invariably induces checking. The loss in weight in a stringer 8 by 16 inches in cross-section and 16 feet long, in three months, varies from 40 to 60 pounds, the loss being proportional in a general way to the amount of sapwood the timber contains. Checking is less serious, however, when the timbers contain a considerable amount of sapwood than when they are practically all heartwood.
(4) The best results are obtained when the air-seasoning is started in the late fall or early winter months. At this time of the year, the air is usually moist enough to prevent rapid drying on the surface, and, in consequence, serious checking.
(5) The absence of shrinkage in redwood timbers is very noticeable, although redwood contains a large amount of moisture when cut. On account of its lowshrinkage factor, it can be seasoned without serious checking.